In one of the known methods of treating therapeutically gastrointestinal inflammatory diseases, especially the gastric ulcer or the duodenal ulcer, the administration of a medicinal compound having an activitiy to inhibit the secretion of gastric acid is performed. The medicinal compound administered for that purpose includes cimetidine (see "Merck Index" 10th Edition, Monograph No. 2254) which is known as an antagonist to the histamine H.sub.2 -receptors. Recently, it was discovered that the secretion of gstric acid is governed by an enzyme, H.sup.+, K.sup.+ -ATPase having the specific property that this enzyme can be activated by potassium cation. It is thus revealed that an inhibitor to said enzyme may be useful as an agent of inhibiting or suppressing the secretion of gastric acid and hence be useful as an agent of treating therapeutically the ulcer in the gastrointestinal tract (see a Japanese medicinal journal "I-gaku no Ayumi" Vol. 128, page 296 (1984); a Japanese medicinal journal "Sa-i-shin I-gaku" Vol. 37, page 481 (1982); the "Nature" Vol. 290, 159-161 (March 1981) and "Drugs" Vol. 25, 315-330 (1983). As example of the known compounds having the activity to inhibit the H.sup.+, K.sup.+ -ATPase is mentioned a group of benzimidazole derivatives which is typically represented by omeprazole (identified as 2-[(3,5-dimethyl-4-methoxy-2-pyridyl)methylsulfinyl]-5-methoxybenzimidazol e (see Japanese patent application first publication "Kokai" No. 141783/79; U.S. Pat. No. 4,255,431; No. 4,337,257 and No. 4,508,905). It is also known that some of the above-mentioned benzimidazole derivatives exhibit an activity to effect the gastrointestinal cytoprotection (see Japanese patent application first publication "Kokai" No. 53406/82; U.S. Pat. No. 4,359,465).
The antagonists to the histamine H.sub.2 -receptors which are typically represented by cimethidine, as well as the inhibitors to the H.sup.+, K.sup.+ -ATPase which are typically represented by omeprazole show a high activity to inhibit the secretion of gastric acid, and owing to this activity, they are able to exhibit good curative effects in the therapeutic treatment of the gastric ulcer. However, it has often been observed that when the ulcer has been cured through the administrations of the aforesaid drug and thus the administration of the drug is stopped, the ulcer is very much likely to return with the lapse of time after the stoppage of the drug administration. Therefore, it is not worthy to say that cimetidine and omeprazole are a fully satisfactory drug for therapeutic treatment of the gastrointestinal ulcers.
Accordingly, it has been a lasting demand to exploit a better antiulcer drug effective in the therapeutic treatment of gastrointestinal ulcers. We, the present inventors, have paid our attention on indole derivatives, and we have made extensive researches in an attempt to provide a class of new indole derivatives which exhibit the activity to inhibit the gastric acid secretion and are useful as an antiulcer agent having such further advantages that long successive administrations of the drug to the patients is allowable and the relapse or return of the ulcer as once healed is effectively prevented even after the stopped administration of the durg. As a result, we have succeeded in synthetizing a class of new indole derivatives which have now been found by us to exhibit a mild activity of inhibiting the gastric acid secretion as well as remarkable cytoprotective effects on the gastrointestinal tract, in combination, as will be demonstrated by the pharmacological experiments shown hereinafter. Thus, we have accomplished this invention.